The rodent trophoblast carries out intense endoreduplication of its total genome in the absence of mitosis or cell division and differentiates into giant polyploid cells during the latter 2/3 of gestation. Trophoblast cells derived from mid-gestation rat embryos thus represent a system composed of normal albeit embryonic mammalian cells which are essentially synchronized in S phase and which are capable of carrying out intense DNA enco-reduplication. Nuclei from such cells appear to represent a unique system in which to study various aspects of mammalian DNA replication in vitro, and the process of polyploidization as well as the role of any cytoplasmic factors which might be involved in the control of cellular replication. We propose to determine quantitatively the types and activities of the DNA polymerases present in rat trophoblast cells from day 11 to day 15 of development in order to ascertain which of the known cellular DNA polymerases may be involved in the process of polyploidization. It is also proposed to develop a system consisting of nuclei from actively replicating 13-day trophoblast cells which will carry out endoreduplication in vitro as a continuation of the process started in vivo. Such a system will be used to assay for potential factor(s) in the cytoplasm of trophoblast cells from day 11 to day 15 of development which either stimulate (initiate) or inhibit endoreduplication by these active nuclei in vitro. Any factors found will be purified and their effect on the pattern of DNA synthesis by both trophoblast nuclei or the isolated DNA polymerases will be investigated.